1. Field of the Invention
The present invention relates to an image forming method to form a toner image on a transfer material by toner containing a resin particle and a release agent having melting point in a specific range and by utilizing ultrasonic vibration.
2. Related Art
A digital system is presently the main current of image forming of a type of an electrophotographic system, and there is given a full-color image forming technology as one of the trends of that technology. As one of the technologies to promote color toner images, there is in existence the one that forms full-color toner images by using oilless toner wherein much release agents are contained in toner particles (for example, TOKKAI No. 2002-214821 (see Paragraph 0049)).
In the digital image forming, a small dot image on a level of 1200 dpi (the number of dots per 1 inch which is 2.54 cm) is sometimes visualized. Therefore, images are formed by using toner that is in a size of several microns called a small particle size toner.
In image forming by a small particle size toner, there has been a tendency that transferability is worsened when toner images formed on the surface of a photoconductor are transferred onto a transfer material such as a sheet of paper or OHP film. In particular the tendency is remarkable in full-color image forming to form toner images by superposing Y color toner, M color toner and C color toner, and thus, it has been impossible to transfer toner images from the photoconductor surface or from an intermediate transfer object stably and surely, which has made it difficult to form color images having excellent color balance and density on a transfer material.
Accordingly, there have been studied technologies to transfer toner images surely onto a recording medium by giving physical operations on the photoconductor, and as one means thereof, there is a technology wherein ultrasonic waves are applied on an image carrier that holds toner images when transferring toner images onto a transfer material, and thereby, toner images are transferred efficiently onto a transfer material from the surface of the image carrier by the actions of vibrations generated from the ultrasonic waves (for example, TOKKAI No. 2001-100546 (see Paragraph 0022) and TOKKAI No. 2001-117381 (see Paragraph 0035).
However, transfer of toner images employing supersonic waves disclosed in TOKKAI No. 2001-100546 or TOKKAI No. 2001-117381 has been one developed for the toner for which oil is coated on a transfer material in the course of fixing. So, image forming was tried through this transfer method by using oilless toner, in vain. When transfer by means of ultrasonic waves was tried by using oilless toner, release agents were removed from toner particles by the actions of vibrations coming from the ultrasonic waves, resulting in the problem that a transfer material wound itself round a fixing roller in the fixing process, and offsetting was caused.
Since externally added agents have also been removed from the oilless toner together with the release agents, toner images were easily disturbed by the influence of vibration by ultrasonic waves, because of the tendency that force of adhesion to the photoconductor is increased and transfer rate is lowered, thus, it was difficult to superpose each monochromatic toner image correctly on a full-color image.